U.S. Pat. Nos. 4,454,801 and 3,022,772, issued 19 Jun. 1984 and 27 Feb. 1962, respectively, and assigned to the assignee of this invention, describe a proportional control valve in motor vehicle power steering gear including a valve spool connected to a manual steering wheel, a valve body connected to steered wheels of the vehicle, and a torsion bar between the valve spool and valve body defining a center position of the former relative to the latter. A plurality of throttling orifices of the control valve defined between the valve body and the valve spool regulate a steering assist boost pressure when the valve spool is rotated relative to the valve body from the center position by manual effort at the steering wheel. Rotation of the valve spool relative to the valve body induces a restoring or centering torque in the torsion bar which is perceived by a driver as the manual effort necessary to steer the vehicle.
U.S. Pat. No. 5,119,898, issued 9 Jun. 1992 and assigned to the assignee of this invention, describes a power steering gear having a proportional control valve as described above and an electromagnetic apparatus for varying the effective centering torque of the torsion bar. The electromagnetic apparatus includes a cylindrical ring attached to the valve spool having a plurality of radial permanent magnets thereon, a pole piece attached to the valve body having a plurality of outer pole teeth outside of the magnet ring and a plurality of inner pole teeth inside of the magnet ring, and a stationary exciting coil magnetically coupled to the pole teeth. An electromagnetic torque is induced between the pole piece and the permanent magnet ring which either increases or decreases the effective centering torque of the torsion bar depending upon the direction of current flow in the exciting coil. A motor vehicle power steering gear according to this invention is an improvement relative to the power steering gear described in the aforesaid U.S. Pat. No. 5,119,898.